In an LTE system, carrier aggregation (CA hereinafter) for enabling communication by simultaneously using a plurality of component carriers (CC hereinafter) is introduced. As shown in FIG. 1, in CA up to Rel-11of LTE, simultaneous communication is performed by using a plurality of CCs under the same base station eNB, so that it is possible to realize high throughputs.
On the other hand, in Rel-12, this is further expanded so that Dual connectivity is proposed in which simultaneous communication is performed by using CCs under different base stations eNB to realize high throughputs (non-patent document 1). That is, in Dual connectivity, the user apparatus UE performs communication simultaneously using radio resources of two physically different base stations eNB.
Dual connectivity is also referred to as Inter eNB CA (inter-base station carrier aggregation), in which Master-eNB (MeNB) and Secondary-eNB (SeNB) are introduced. FIG. 2 shows an example of Dual connectivity. In the example of FIG. 2, an MeNB communicates with the user apparatus UE by a CC#1, and the SeNB communicates with the user apparatus UE by a CC#2 so that Dual connectivity is realized.
In Dual connectivity, (one or a plurality of) cell(s) under an MeNB is called MCG (Master Cell Group), and (one or a plurality of) cell(s) under an SeNB is called SCG (Secondary Cell Group). A cell of the SCG added first is called a PSCell (primary SCell). Although an SCell in CA is activated and deactivated by a MAC control signal in general, it is assumed that the PSCell is always activated.